It is conventionally known to execute a part program by means of an NC automatic programming system to automatically prepare an NC program. A conventional programming system is arranged to execute a part program from the beginning to the program end for NC program preparation, in response to manual input of an execution command. The NC program thus prepared, i.e., the result of execution of the part program, is checked by an operator, and if an error exists in the part program, the erroneous portion of the part program is modified/edited by the operator. Then, the modified/edited part program is executed again by the programming system from the beginning to the end of the program.
According to the conventional method described above, it is difficult to learn the result of the execution of each of program statements constituting the part program, and much effort is required to find an erroneous portion of the part program. Moreover, since the modified/edited part program is re-executed from the beginning to the end thereof, the execution time of the part program is long, and therefore, much time is required for the preparation of an NC program.